class_docstr: A highly optimized equivalent of lambda x: Entmax15([x, 0]) 
code: def _backward(output, grad_output):
      gppr0, gppr1 = output.sqrt(), (1 - output).sqrt()
      grad_input = grad_output * gppr0
      q = grad_input / (gppr0 + gppr1)
      grad_input -= q * gppr0
      return grad_input
  
imports: from torch import nn
  from torch.autograd import Function
  import torch.nn.functional
  import torch
label_desc: Use image node_id 4 for calling the Entmoid15 obj's underlying member method code with example usage: obj._backward(output, grad_output) and returns: grad_input